Technical Field
This disclosure relates in general to redox flow battery (RFB) systems for energy storage, and in particular to the so-called all-vanadium RFB system. This disclosure addresses the problem of accidental oxygen discharge on a carbon base, positive electrode during charging and of undue precipitations in an overcharged positive electrolyte solution.
Related Art
RFB energy storage systems [1-7] are recognized as particularly efficient and flexible candidates for large scale energy storage requirements of intelligent power distribution networks being developed.
The all-vanadium (V/V) RFB system using the redox couples V+2/V+3 in the negative electrolyte solution and V+4/V+5 in the positive electrolyte solution is probably the one that has had significant industrial applications that is still most extensively studied. Other similar RFB systems like Fe/V, V/Br, Cr/Fe, Zn/Ce, Polysulfide/Br, have been studied but have not had a comparable commercial acceptance. A common feature to these systems is that, for economically acceptable current densities to be supported, porous and fluid permeable electrodes are necessary. Moreover, chemical inertness of the electrode materials that need to be retained when switching from cathodic polarization to anodic polarization during a cycle of charging and discharging of the redox storage system, and the requisite of having a relatively high H+ discharge overvoltage when negatively polarized in respect to the electrolyte solution and a high OH− discharge overvoltage when positively polarized in respect to the electrolyte solution, obliges to use carbon base electrodes.
Yet, preventing parasitic OH− discharge and/or H+ discharge in case of localized depletion of oxidable and reducible vanadium ions of the respective redox couples in the two solutions because of non uniform mass transport and/or electrical potential throughout porous electrode felts of non woven activated carbon fibers, generally sandwiched between the ion permeable cell separating membrane and the surface of a conductive current distributing plate, remains a critical aspect.
Parasitic oxygen discharge at the carbon electrode may accidentally becomes the main current supporting anodic reaction if the design maximum current density limit is for some reason surpassed or if the charging process is accidentally protracted beyond full vanadium oxidation in the positive electrolyte solution to V+5. In the latter event, another serious effect may start to manifest itself, notably a gradual precipitation of vanadium pentoxide according to the reaction: 2VO2++H2O═V2O5+2H+.
The first of these hazardous occurrences may lead to a rapid destruction of the carbon felt and of the carbon-based current collecting plates by nascent oxygen with generation of CO and CO2. For this reason many substances have been identified as poisoning agents of oxygen evolution on carbon anodes in the typical sulphuric acid electrolyte solutions of vanadium RFBs like antimony (Sb+3), Borax and tellurium (Te+4), generally preferred because besides raising the oxygen evolution overvoltage, they also poisons H+ discharge in case of migration/contamination of the negative electrolyte solution. The second occurrence, if unchecked, causes clogging, particularly difficult to remedy, specially in the pores of the carbon felt electrode and unbalancing of the electrolytes. As it is well known, parasitic hydrogen evolution in a vanadium RFS energy storage cell may be favoured by accidental contamination of the electrolyte solutions with metals having a low hydrogen overvoltage like Fe, Ni, Co, Pt, Pd . . . etc. that may deposit on the carbon electrode structure, and/or when V+3 has been completely reduced to V+2 in which case the only electrode reaction that may support circulation of electric current becomes the electrolysis of water.
Specific monitoring of working conditions in the cells is indispensable and its shortcomings have been the cause of costly failures. More sophisticated and reliable ways of controlling the operation of RFB energy storage systems are been developed.
Prior patent application No. PCT/IB2012/057342, of the same applicants, discloses a reliable monitoring system of the operation conditions that provides a long sought detectability at single cell level, impossible with the multi-cell bipolar stacks typical of known industrial all-vanadium flow redox batteries. The content of his prior patent application is herein incorporate by express reference.